Every model is backtested on held-out history against two references: a naive persistence forecast and the method you use today. If it does not beat both, by a margin that matters to your imbalance costs, we say so and stop. We report accuracy separately for the days that hurt (holidays, heatwaves, cold snaps, sudden cloud) because averages flatter forecasts on the easy days.
In production, forecast error and calibration are monitored daily, and a drift alert goes to a person when the inputs or the errors move outside their recent range. Where forecasts or the data behind them could be inside information under REMIT, access and logging are designed with your compliance team.